Low cost lasers which can operate in the continuous wave (CW) mode are of great interest and used in consumer electronics, medical, sensing, telecommunication and other industries. Each application uses particular wavelengths and as such lasers which can provide different emission wavelengths are of particular interest.
Constructing a tunable laser is very difficult for a variety of reasons and is the subject of a significant amount of research in the laser science and technology field. For tunable lasers, typically dye lasers are used. These lasers are based, typically, on fluorescent dyes dissolved in an active medium. The system is then placed in an external cavity and optically pumped with a light source to achieve lasing. The emission energy from a dye laser can be significantly enhanced by using a secondary amplifier. Amplifiers are systems or media made of materials similar to the laser material, and are also optically pumped similar to the laser material but are not within a cavity. As such, their main purpose is to amplify a pre-existing laser emission. Clearly, for efficient operation, the emission of the laser from the cavity must match the gain spectrum of the amplifiers. These systems have been extensively studied and used in a variety of fields ranging from spectroscopy to medicine.
A drawback of dye lasers is the initial alignment of the cavity. Since dye lasers provide a broadband emission, it is difficult to obtain a single laser wavelength and avoid artifacts such as mode hopping. To achieve single wavelength operation, the laser cavities are typically complicated systems with several optical elements, such as a grating or etalons, designed to allow the user to isolate the laser emission. This drives up the cost and makes the laser bulky. More importantly, this means that the operator must become very familiar with the system and tune it for different wavelengths. This has made these systems not applicable in industries where the operator is not a laser scientist or engineer.
To avoid this issue, fixed wavelength diode lasers are used. In this case, a compact laser system can be created out of semiconductor lasers. The cavity is built upon the semiconductor directly by processes such as vacuum deposition. Diode lasers operate by applying a voltage across the material and are typically electrically pumped. Diode lasers have the drawback that they can only be operated at select, predetermined wavelengths and are not used in conjunction with amplifiers. This has limited their use to low power applications. To alter the wavelength, the complete laser system has to be changed which is a significant drawback for several applications. Despite these shortcomings, they remain the cornerstone of many consumer electronics.
Distributed feedback diode lasers were designed to overcome this problem. In these systems, the material itself acts as a cavity with a wavelength selectivity built in. In such a case, low threshold lasing can occur at the select wavelengths. However, these systems are expensive to fabricate and remain single wavelength with limited capability for true amplification.
Cholesteric liquid crystals offer the ability to create a distributed feedback system intrinsically by creating a helical structure that acts as a polarization dependent Bragg reflector and a distinct laser cavity. In these systems, incident light with wavelength and handedness matching that of the structure cholesteric liquid crystal is reflected. Cholesteric liquid crystals (CLC) which exhibit this behavior are referred to as Photonic Band Gap (PBG) materials. Analogous to semiconductors, the reflection properties can be altered if there is a “defect” in the structure. In dye doped PBG systems, it is possible to get directional emission and lasing without the need for conventional external laser cavity. Lasers made from this concept are referred to as distributed feedback lasers or as photonic band gap lasers. The efficiency of these lasers depends on a number of parameters including the dye, chirality of the host and the birefringence of the liquid crystal. A parameter of the system that is used to characterize the efficiency of the PBG is the “density of the states” at a particular wavelength. A larger density of the states results in a lower threshold in lasing in the materials. In CLC based PBG, this jump in the density of the states occurs at the edge of the reflection band, consequently lasing is typically observed at those locations.
Several efforts have targeted the optimization of photonic band gap materials to reduce the threshold for observation of lasing (see Genack et. al., US APPL. 2002/0003827, and Kopp et al. U.S. Pat. No. 7,142,280). Local changes in the density of states have been suggested as means of reducing the threshold with some success (see L. M. Blinov and R. Bartolino Ed. “Liquid Crystal Microlasers”. Transworld Research Network, Kerala, 2010). These approaches mimic those that have been used for diode lasers based on semiconductors. However, in the case of CLC-based PBG, the intrinsic nature of the material, coupled with use of dyes allows for low cost, large area lasing. In addition, since the active medium is a dye, both wavelength tunability and amplification are possible. These properties can help overcome many of the issues associated with the diode laser. As such, the concept of using cholesterics as a laser cavity has become subject of great research in the academic world. A review of these research efforts is provided in, for example, H. Coles and S. Morris, “Liquid-crystal lasers”, Nature Photonics 4, 676-685 (2010); and in Furumi S. “Recent Progress in Chiral Photonic Band-Gap Liquid Crystals for Laser Applications” The Chemical Record 10, 394 (2010).
Dye-doped cholesteric liquid crystal (CLC) lasing has been observed in monomeric, oligomeric, polymeric CLCs, elastomers and even blue phases of liquid crystals. Lasing in circularly polarized mode matching the chirality of a CLC helix occurs at a band-edge of a CLC spectral reflectivity profile. However, to date, all CLC laser observations have been based on pulsed excitations by nanosecond or picosecond laser sources. Tunable, continuous wave (CW) CLC lasers are greatly desired but, despite many efforts they have not been realized. For example, Coles and S. Morris conclude their review of the field by stating that “The ultimate aim is to achieve pumping using low-power incoherent optical excitation. At present, pumping is restricted to high-intensity optical pulses of short duration”, therefore “the threshold for lasing must be further reduced before a low-power incoherent light source can be used. We must not understate the prospect of an all-organic device that is compact (millimetre thickness), wavelength-tunable and quasi-continuous-wave. This Review has highlighted the significant steps that have already been taken towards this goal, but these properties have yet to be demonstrated simultaneously in a single device.” (H. Coles and S. Morris “Liquid-crystal lasers”, Nature Photonics 4, at 685).